The present invention relates to a light-generating arrangement which comprises at least one supporting means in which a light-generating solid body is arranged, and to a method for arranging optical components in a supporting means.
An arrangement for generating coherent light in the form of a solid-state laser consists in its simplest embodiment of a light-generating, solid body of a host material (e.g. glass or crystal) doped with active ions (e.g. rare earth metals or transition metals) and a set of mirrors which define an oscillator cavity round the light-generating body. The light-generating body is usually supplied with the requisite energy optically, so-called optical pumping. Such pumping is usually effected by means of a flash lamp or a laser diode arrangement.
Another arrangement for generating coherent light is a so-called optical parametric oscillator (OPO). In an OPO the light-generating body comprises a non-linear crystal. An OPO does not generate light in the same way as a laser, but functions as a converter from one light wavelength to another. The pumping of an OPO is carried out by pumping coherent light, which has been generated in some other way, into the OPO, whereby coherent light is generated at other wavelengths. Other arrangements for wavelength conversion function in a similar way. Examples of different types of wavelength conversion are frequency doubling, sum frequency generation, difference frequency generation and parametric frequency generation.
A holding device for a solid laser body is previously known from, for instance, DE-196 43 531 A 1. This holder uses a foil between the laser body and the holder for increasing the heat transfer from the laser body to the holder. Recesses are made in the holder for receiving superfluous foil material, for instance, in connection with expansion of the laser body. Thus, the aim is to reduce internal stress in the laser body.
U.S. Pat. No. 5,265,113 discloses an integrated microsystem for electronic and optical components which are mounted in anisotropically etched structures in a base plate of semiconductor material. The components are electrically controllable and movable in such a manner that their positions relative to the base plate are actively adjustable.
The above-mentioned arrangements for generating coherent light have, however, limitations as regards the light power which can be extracted from a light source of a certain volume. In connection with optical pumping of coherent light sources, the efficiency is lower than 100%. This means that some of the energy which is deposited in the light-generating body of the arrangement is lost in the form of heat. Since arrangements for generating coherent light, which are based on solid, light-generating bodies, such as solid-state lasers and arrangements for frequency conversion, are driven towards higher and higher output power, problems often arise as regards the heat transfer from the light-generating body because of the low heat conductivity of the dielectric materials of which the light-generating body typically is comprised. The heating which thus arises results in thermal expansion of the heated area and other thermal effects, such as thermal lensing, thermal birefringence and reduced gain (caused by, inter alia, a decrease of the lifetime of the excited state in the active ions of the laser material and/or an increased thermal population of the energy levels of the active ions).
Furthermore, there is a risk that, for instance, the laser material of a solid-state laser cracks as a result of thermally induced internal stress. This is a particularly great problem as regards laser material with an anisotropic atomic structure.
For the non-linear crystal in an OPO or some other frequency-converting arrangement, the main problems consist of thermal lensing and poor phase matching caused by the heating.
The miniaturisation of coherent light sources of the type mentioned above involves a reduction of the volume which is occupied by the light beams in the light-generating body. In order not to increase the thermal loading, the power of the light source has to be decreased. This is a considerable limitation of prior-art technique since both a miniaturisation of the light source and an increase of the output power usually are desired in one and the same light source.
The problems mentioned above have limited the output power from miniaturised, coherent light sources to be typically a few hundred milliwatt.
An object of the present invention is to provide a construction for optically pumpable light sources by means of which the handling of the thermal loading on the light-generating body as well as the alignment of components included in the light source are simplified to a considerable extent.
Another object of the invention is to provide a manufacturing method which allows mass manufacturing of optically pumpable light sources according to the present invention and which allows simple incorporation of further components in the arrangement. Examples of further components which advantageously are incorporated in the arrangement are so-called functional elements. A functional element should, as regards the present application, comprise non-linear crystals for frequency conversion, active or passive Q-switches for producing light pulses with high peak power, as well as active or passive mode locking means to produce ultrashort light pulses. The term xe2x80x9cfunctional elementxe2x80x9d also comprises means for external intensity modulation or phase modulation and means for controlling the direction of the emitted light beam.
According to the invention, a solid-state laser or an arrangement for wavelength conversion is thus provided which in its simplest embodiment is characterised by an optically pumpable, light-generating body of a dielectric material, the body being arranged in a supporting means and having a shape which is substantially complementary to a guiding structure formed in the supporting means. The guiding structure is formed with a high degree of accuracy, for instance, by etching the supporting means or by replicating an original. Between the light-generating body and the guiding structure of the supporting means, a thin contact layer is arranged, the purpose of which is to improve the adherence to and/or the heat transfer to the supporting means. Due to the fact that the contact layer consists of a deformable material, any discrepancies as regards complementarity between the guiding structure and the light-generating body will be filled by the contact layer, whereby a close fit is obtained between said complementary structures. Preferably, the contact layer has a thickness less than 100 micrometer. A thickness of some tens of micrometers is especially preferred.
Examples of optically pumpable, light-generating solid bodies of dielectric material which might constitute the light-generating body of the invention, are optically non-linear crystals for frequency conversion and laser material based on crystal or glass.
According to another aspect of the invention, a method for manufacturing the above-mentioned light sources is provided. Briefly, the method is characterised in that a plate of a crystalline material (a supporting means) is provided with one or more guiding structures which have a shape that is substantially complementary to the light-generating body. Alternatively, a supporting means is provided with guiding structures by replicating an original. In the guiding structure a contact layer of the type mentioned above is arranged, preferably by vapour deposition, electroplating or sputtering of a jointing metal, after which the light-generating body is arranged in the guiding structure of the supporting means. According to a preferred embodiment, the supporting means comprises at least two parts which jointly enclose the major part of the area of the side faces of the light-generating body, while two opposite end faces of said body are let free for the passing of light. The supporting means is conveniently mounted in thermal contact with a thermoelement with the aim of allowing control of the temperature of the light-generating body. The supporting means may also be provided with microchannels for further increase of the possibility of thermal control.
One advantage of the present invention is that the light-generating body fills the entire guiding structure. This embodiment gives together with said contact layer an excellent heat transfer from the light-generating body to the supporting means.
Another advantage of the invention is that, thanks to the supporting means being micromechanically provided with well-defined guiding structures, the device can with a high degree of accuracy be miniaturised to millimetre dimensions. The limitation of the heat transfer from the light-generating body is usually the limited thermal conductivity of the light-generating body. A reduction of its cross-sectional area thus results in a substantial reduction of the thermal loading on the arrangement. The lower limit of the cross-section is determined by the cross-section of the light beam which propagates through the arrangement.
The term xe2x80x9cminiaturised light sourcexe2x80x9d relates mainly to bulk lasers and arrangements for frequency conversion, whose optically pumpable light-generating body has a cross-sectional area, perpendicular to the propagation direction of the light, which is in the range of less than 1 mm2 to a few mm2, but the invention also relates to other light-generating bodies, for instance, waveguiding structures.